Field
This disclosure relates to memristor devices and to memristor devices suitable for implementation of neuromorphic networks.
Description of the Related Art
A memristor is a two-terminal electronic component having a resistance that is not constant, but rather is determined by the history of the voltages applied to, or the currents that flowed through, the device. A memristor device may be used in a binary mode in which voltage pulses of opposing polarity may be used to switch the device to either a “on” state (i.e. a low resistance state) or “off” state (i.e. a high resistance state). The state of the device may then be interrogated by applying a voltage less than a threshold voltage needed to cause the device to change state and measuring the current flow through the device. Arrays of memristor devices operated in the binary mode have the potential to provide very dense nonvolatile digital memories.
Some memristor devices may be operated in an analog mode in which the resistance of the devices may be set to any value between a minimum (i.e. fully “on”) and a maximum (i.e. fully “off”) value. The ratio of the maximum resistance to the minimum resistance may vary by four orders of magnitude or more. Arrays of analog memristors may be used to implement neuromorphic networks which, in over-simplified terms, are electronic networks that mimic the operation of neurons.
Memristors have been fabricated using a variety of switching materials, which is to say materials that are capable of switching between high and low conductivity states. Many of these memristor devices require “forming” (also called “electroforming”) before the devices can be switched. To form a memristor, a current-limited DC voltage may be applied across the memristor. The applied voltage may be gradually increased to a level, called the “forming voltage”, where an abrupt increase in current thought the memristor occurs. Equivalently, a DC current may be forced through the memristor. The current may be gradually increased until the voltage across the memristor reaches the forming voltage, where an abrupt drop in the voltage across the device occurs. With either technique, forming is a one-time operation after which the memristor exhibits switching behavior as previously described. The exact mechanism of forming may not be the same for all switching materials, but may involve aligning or aggregating dopants or other conductive species within the switching material.
A memristor within a memristor array may be formed as described in the previous paragraph by applying the forming voltage between the appropriate row and column electrodes with the unused row and column electrodes floating. During forming, a fraction of the forming voltage is applied across every other memristor in the array. The current flow through individual unformed memristors and formed memristors switched to their “off” states may be small compared to the current flow through the memristor being formed. However, the current flow though any formed memristor in the “on” state may be substantial and possibly high enough to cause damage to the memristor array. To allow one memristor in an array to be formed without inadvertently turning on other memristors in the array, the forming voltage of each memristor is preferably not more than about two times the voltage required to switch a previously-formed memristor from the off to on states.
Memristor devices that require higher switching voltages may be used in memristor arrays if some form of select device is integrated in series with the memristor. The select device may be, for example, a diode or a switching transistor.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having a reference designator with the same least significant digits.